1980 300D A/C problem

Hoping someone can help me diagnose an A/C problem with this car. For background, the A/C system was mostly replaced a few years ago (new compressor, dryer), and the monovalve is reatively new as well.
My A/C no longer turns on. If I push the auto-hi or auto-low buttons, I get no A/C and no blower. Same for the bi-level button. But if I hit the defrost button, the blower and the A/C come on, and it blows cold for a few minutes. If I then hit the bi-level button the fan blows, but no A/C, but very rarely the A/C will stay on. I will occasionally get the fan to work with with Auto Hi or Auto Low, but usually not.
Given that my car has relatively new components, is the problem most likely the push buttons on the dash? If so, is this something I can easily repair/replace myself? Could it be the A/C compressor switch on the dash? Does defrost override this switch?
Hoping for some help,
Rip
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Your refrigerant level is low... Since you changed the components a few years ago... it probably is R134a... if so, just go to auto store and pick up a gauge and a couple of bottle... you probably only need 1 to recharge. Pick up one that has sealer in it... and other regular.
With engine running and A/C on... see if your A/C compressor is running... if not, squirt some frigerant in until it hits about 70 PSI or so... until it kicks on...
Once it kicks on... make sure you aux fan is running also... if everything is running, then the pressure should start to drop... it should be at about 30 to 35 PSI..
You are done and it should be cold air coming out of the vents... Make sure all windows are shut when you do this procedure.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Yes, it's been converted to 134A. I can easily try to charge it. But if the refrigerant were low, why would it blow cold when the defroster is turned on?
Rip
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I had similar problems with my '77 300D. I ended up having to replace the climate control servo unit. This part is under the hood on the passenger fender and has many vacuum hoses/wires and a couple heater hoses going to it. It ran $330 five years ago but I cannot find it listed in current online catalogs. The part number for mine was 000-830-06-84. An additional symptom of mine was sporatic heating. Replaced it and my ACC system worked perfectly. Good luck finding your specific problem. DD
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
I've just been working on my '80 300SD's climate control. I've owned the car for 25 years and have some experience that may help you. The same thing once happened to my car.
Your car's climate control system has a "cold engine lockout" feature that prevents it from operating (except in Defrost mode) until the coolant is warm enough to provide heat. This is done with a bi-metallic vacuum switch that's attached to the VERY bottom (beneath the hoses) of the servo (that black mushroom to the right of the engine). When the coolant warms, the thermo valve opens and vacuum is applied to a vacuum power ON switch for the blower; vacuum is then also available for the duct door motors etc.
The specific problem in your car is either (i) a vacuum leak or (ii) the bimetallic vacuum switch is bad. The first possibility is much more likely than the second so check the vacuum connections, especially those at the bottom of the servo (and that the bimetallic switch is tightly attached to the servo). You can bypass the cold engine lockout by coupling its yellow and black vacuum lines; just insert the yellow into the short piece of hose on the end of the black vacuum line. Done.
My current project is to replace this old analog system with a digital retrofit. The $699 retrofit is sold by PerformamceProducts.com and built by UnlimitedTools, a company in Flagstaff AZ. It's not done yet but I'm very hopeful. Good riddance to the Amplifier (logic board) and the Servo!
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
1) you DON'T have a mono valve.
2) you have a vacuum leak in your dash for the ACC system.
This is why you get full blower and max heat [the compressor turns on in defrost mode] in defrost. Federal requires this to happen when the vacuum fails. When you press defrost you hear the relay click behind the glove box. This bypasses both the blower vacuum valve and the compressor vacuum valve.
You need to find which vacuum element gave up. More than likely, it is more than one.
Do NOT use the Mossberg repair technique..... loading up the shotgun and throwing new EXPENSIVE parts at it. This will break you before you fix it!
99% of your ACC is vacuum. Vacuum controls just about all!

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.